Propionic acidemia is an inherited disorder of protein metabolism which is associated with recurrent bouts of vomiting and lethargy, metabolic keto-acidosis and hyperammonemia, usually requiring admission to the hospital. The aim of the study is to evaluate intestinal transit in patients with propionic acidemia and study the relationship between intestinal transit and metabolic stability. The study will involve the measurement of intestinal transit with the use of a lactulose breath test and of radiopaque markers to calculate total oro-anal transit.